One known control system based on a sliding mode control theory is disclosed in Japanese Patent Laid-open No. Hei 9-274504, for example. The publication proposes a method of setting a hyperplane in the sliding mode control theory according to the convergence state of a controlled state quantity. According to the proposed method, the convergence response and convergence stability of the sliding mode control is improved.
For controlling a plant, which is a controlled object, with the sliding mode controller, it is necessary to produce a model of the plant and determine model parameters representing the characteristics of the model of the plant (i.e. the controlled object). The model parameters may be set to predetermined constant values. However, the values of the model parameters usually change due to aging and disturbance. Therefore, it is desirable to use a model parameter identifier for identifying the model parameters on a real-time basis and carry out the sliding mode control using the model parameters that are identified by the model parameter identifier.
The model parameter identifier detects an identifying error which is a difference between the output of the plant which is calculated using the identified model parameters and the actual output of the plant, and corrects the model parameters in order to eliminate the identifying error. Therefore, regarding the model parameter identifier, the following problems may occur.
Due to nonlinear characteristics and disturbance whose average value is not “0”, the identifying error does not become “0” even though substantially optimum model parameters have actually been obtained. Therefore, the model parameters which do not need to be corrected are occasionally corrected. As a result, a drift occurs in which the values of the model parameters gradually shift from their optimum values to some other values to make the control performed by the sliding mode controller unstable.
Conventionally known is an air-fuel ratio control system in which a controlled object model is defined by modeling a plant, i.e., a controlled object, and model parameters of the controlled object model are identified by a parameter adjusting device (for example, Japanese Patent Laid-open No. Hei 11-73206). According to this system, the air-fuel ratio of an air-fuel mixture to be supplied to the internal combustion engine is feedback-controlled to a target air-fuel ratio by a self-tuning regulator using the identified model parameters.
In the above control system, each model parameter is calculated by adding an initial value of the model parameter and an updating component which is calculated according to the identifying error of the model parameters.
In the above control system, the initial values of the model parameter is determined irrespective of the state quantity of the plant at the time of calculating a control input to the plant. Therefore, the time period which is necessary for the model parameters to converge on their optimum values becomes longer when the characteristics of the plant change in a relatively short time period, which results in insufficient controlling performance.
One known control system based on the sliding mode control theory is disclosed in Japanese Patent Laid-open No. Hei 9-274504, for example. The publication shows that the stability of the sliding mode control is determined, and when the sliding mode control is determined to be unstable, the value calculated before the sliding mode control is determined to be unstable is used for a control input to the controlled object.
Another control system is known from Japanese Patent Laid-open No. Hei 11-93741. In this control system, the stability of the sliding mode control is determined, and when the sliding mode control is determined to be unstable, the control input to the controlled object is set to a predetermined value.
However, according to the above-described conventional control systems, when the sliding mode control is determined to be unstable, the control input to the plant is fixed, which makes it impossible to converge an output of the plant to the control target value.
In the control system shown in Japanese Patent Laid-open No. Hei 11-73206 described above, the model parameters are calculated by adding updating components, which are calculated according to the identifying error of the model parameters, to the initial values of the model parameters.
According to this control system shown in Japanese Patent Laid-open No. Hei 11-73206, a drift of the model parameters due to disturbances is prevented. However, since the model parameters calculated by the parameter adjusting device are used per se in the self tuning regulator, there remains a room for improvement in maintaining stability of the self tuning regulator.
A control system in which model parameters of a controlled object model which are obtained by modeling a plant, is identified on a real-time basis, and the plant is controlled using the identified model parameters, is shown in Japanese Patent Laid-open No. 2000-179384.
The control system shown in this publication includes an identifier for identifying the model parameters, an estimator for estimating an output of the plant, and a sliding mode controller which calculates a control input to the plant with the sliding mode control, using the model parameters identified by the identifier and the plant output estimated by the estimator. A difference between an output of the plant and a target value of the plant output, and a difference between an input of the plant and a reference value, are input to the identifier and the estimator. The reference value is variably set according to a manipulating quantity for manipulating the input of the plant. By setting the reference value variably according to the manipulating quantity, an effect of raising a converging speed of the control for converging the plant output to the target value, is obtained.
According to the control system shown in Japanese Patent Laid-open No. 2000-179384, a method for setting the reference value according to an adaptive control input which is calculated by the sliding mode controller, is adopted. Accordingly, this method for setting the reference value is applicable only to a control system using the adaptive sliding mode control. Therefore, a reference value setting method which is widely applicable is desired.
One known throttle valve opening control system for a vehicle is disclosed in Japanese Patent Laid-open No. Hei 8-261050. In this control system, an opening of a throttle valve which is actuated by a motor is controlled with a PID (Proportional, Integral, and Differential) control, and control constants of the PID control is set according to an operating condition of the vehicle.
Since the throttle valve actuating device, which is a controlled object, has non-linear characteristics, a sufficient control performance in control accuracy, control stability, and a follow-up characteristic (convergence characteristic) of the throttle valve opening to a target value, is not obtained by the conventional PID control.